The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal flexible connecting member and nerves. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces experienced by the spinal column. A vertebral canal containing the spinal flexible connecting member and nerves is located behind the vertebral bodies.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain as well as diminished range of motion and nerve function. These spinal disorders may also threaten the critical elements of the nervous system housed within the spinal column.
A variety of systems have been developed to stabilize and correct spinal deformities. Many of the systems achieve immobilization by implanting artificial assemblies in or on the spinal column. Lateral and anterior assemblies are typically coupled to the anterior portion of the spine. Posterior implants generally comprise pairs of rods that are aligned along the axis with which the bones are to be disposed. The rods are typically attached to the spinal column by anchor members, such as hooks coupled to the lamina or to the transverse processes, or screws inserted through the pedicles.
One problem with surgically accessing the spine to deal with these disorders is that the skin and tissue surrounding the surgical site must be cut, removed, and/or repositioned to gain access to the location where the devices are to be installed. This is particularly true when installing posterior implants designed to facilitate fusion at various levels of the spine. For example, to install a fixation rod between two pedicle screws, a first pedicle screw is typically secured within the patient's body at one level of the spine and a second pedicle screw typically secured at another level. Sometimes separate access holes or channels are established for each screw and the fixation rod is then maneuvered within the patient's body through the tissue between the two screws. The tools and/or space required to properly position the fixation rod may require significant cutting or repositioning of skin and tissue, which often results in damage, scarring, and longer recovery times. Often, three or more incisions may be necessary for implantation of pedicle screws and the fixation rod.
Various minimally invasive surgical systems have been developed to address these concerns. For example, U.S. patent application Ser. No. 11/228,958 (“the '958 patent”), entitled “Apparatus and Method for Minimally Invasive Spine Surgery” and assigned to the assignee of the present invention, discloses a system in which at least two docking members are configured to align transverse bores provided in two associated pedicle screws. FIG. 9 of the '958 application illustrates an embodiment in which the docking members include a bore or lumen for guiding a therapeutic device, such as a fixation rod, to the pedicle screws. Other approaches to reducing trauma include inserting expandable retractors through a relatively small incision on the patient's body. Once expanded, such retractors may provide access to more than one vertebral level.
These systems and methods may help reduce the amount of manipulation or disruption of tissue. Additional minimally invasive techniques are highly desirable because they may reduce blood loss and scarring, and result in less post-operative pain and shorter recovery times.